Recently studies in these laboratories have revealed that cationic cyclizations initiated by acylnitrilium ions and episulfonium ions represent unusually powerful methods for the convergent elaboration of complex ring systems. Acylnitrilium ion initiated cyclizations not only permit the rapid construction of unsaturated azacycles but also result in the concurrent installation of peripheral 2-acyl moieties. Episulfonium (and episelenonium) induced carboannulations have proven to be quite general in scope and have led to the synthesis of structurally intricate ring systems with the net incorporation of a manipulable functional group (e.g., an ArS or ArSe moiety). The research proposed in this grant application is concerned with the application of these new synthetic methods to the preparation of selected natural target structures. The explicit goals of this research will be to probe the scope and limitations of these new methods in the context of total synthesis. In this way, the utility of these reactions for performing operationally practical chemical syntheses will be revealed.